Active vibration isolation systems comprising a plurality of pneumatic isolators for isolating a load from vibration are known. Such systems typically include three servo-valves for admitting pressurized air to, or releasing pressurized air from, three respective groups of pneumatic isolators based on changes in position of the top surface of the isolators relative to the surface on which the load is supported due to changes in the load. Such a change may occur, for instance, when the load is a stamping press and the latter goes from an idle cycle to a stamping cycle. Thus, as used herein, "change in the load" means a change in position of the center of mass of the load or any other movement of the load which generates a force having a vertical component. The quantity of pressurized air added to or released from the pneumatic isolators is controlled, as a function of the design and adjustment of the servo valves, so that the position of the load remains substantially constant, regardless of changes in the load.
To achieve stability in a vibration isolation system using pneumatic isolators, it is important that the pneumatic isolators be damped. Moreover, the degree of damping required increases as a function of the amount of feedback provided by the servo valve system.
One active vibration isolation system which uses damping to improve the stability of the system is manufactured by MRAD Corporation of Woburn, Mass. and is identified by model number D11020. The MRAD system includes three groups of pneumatic isolators for supporting the load to be isolated from vibration. The MRAD system also includes a servo valve system, including three servo valves, for controlling the height of the load by admitting or releasing air from a corresponding respective one of the groups of pneumatic isolators.
The MRAD system further includes a damping system for damping movement of the pneumatic isolators comprising a tube mounted in each of the airbag springs so as to extend perpendicular to the surface on which the load is supported. Each tube is closed off at its ends and includes an orifice coupling the interior of the tube with the interior of the airbag spring. As pressurized air is added to or released from the interior of the airbag spring, air will flow into or out of the interior of the tube via the orifice therein until the air pressure in the interior of the tube stabilizes with that in the interior of the airbag spring. This flow of air into and out of the interior of the tube damps movement of the airbag spring.
Although the internal tube damping arrangement of the MRAD system provides satisfactory damping under certain conditions, under other conditions the amount of damping provided by the system is inadequate. When the quantity of air admitted to or exhausted from the airbag springs during a given relatively short period of time is relatively large, the orifice in the tube is not sufficiently large to pass the quantity of air required to obtain meaningful damping. If the size of the orifice is enlarged to obtain satisfactory damping during times of large exchange of air, then virtually no damping is provided during times of small exchange of air. Thus, the MRAD system provides a relatively small amount of feedback which reacts relatively slowly to changes in the load.